The optical waveguides typically formed in SOI-based structures comprise silicon (with a refractive index of approximately 3.47), the silicon formed in a “slab”, “rib” or “strip” geometry on the surface of the SOI substrate. A strip waveguide typically comprises crystalline silicon, where crystalline silicon is known to exhibit less loss that a polysilicon form of the same material. However, strip waveguides tend to exhibit relatively high optical loss through their (relatively rough) sidewalls, which serve as scattering surfaces for a propagating optical signal. That is, since an etching process is used to form the sidewalls of the strip, the etchant tends to roughen the exposed sidewall surfaces. Any geometry that introduces sharp corners in the strip waveguide also serves to introduce scattering centers and increase optical loss.
A “rib” waveguide structure conventionally comprises a polysilicon rib formed over the surface silicon layer (often referred to as the “SOI layer”) in SOI-based applications. The bulk losses of polysilicon are greater than that of crystalline silicon, resulting in higher optical losses along the rib waveguide.
One prior art attempt to address the loss experienced by these optical waveguides is discussed in U.S. Pat. No. 6,850,683, issued to K. K. Lee et al. on Feb. 1, 2005. In the Lee et al. arrangement, a post formation oxidation process is used at a high temperature to smooth the rough sidewalls of a silicon waveguide. While this method is somewhat successful in reducing scattering losses, it cannot be used in applications involving “active” waveguide structures, since such a high temperature process will result in unwanted dopant migration within the SOI layer.
An alternative method of creating low loss silicon waveguides is disclosed in U.S. Patent Application Publication 2005/0158002, published for J. A. Kubby et al. on Jul. 21, 2005. In the Kubby et al. arrangement, a silicon nitride cladding layer is formed over a silicon rib waveguide to entrap the propagating optical signal and minimize scattering losses through the sidewalls of the rib structure.